table with backticks

(Thanks to Lu Jingdong, though I did not take his patch directly, as
it contained a significant flaw.)

It wasn't a backtick/parsing problem.  We merely didn't anticipate
and allocate enough space to handle the optional "#mysql50#" table-
name prefix. 

Now, allocate that extra space in case we need it when we look up 
a legacy table to get its file's name.